The present invention relates to an elevator installation with a measuring system for determining the absolute car position of an elevator car movable along at least one guide rail.
In elevators, the positional information is applied in coded form in stationary position along the entire travel path of the elevator car and is read off in coded form by means of a code reading device and passed on to an evaluating unit. The evaluating unit prepares the read-off coded positional information to be comprehensible by a control and derives therefrom data signals which are passed on to the elevator control as so-termed shaft data.
An absolute measuring system with high resolution for determination of the relative position of two parts movable relative to one another is known from the German patent document DE 42 09 629 A1. In hitherto conventional manner, an absolute code mark pattern in the form of a gapless sequence of equal-length code marks of a pseudo random coding are formed there at a first part in a first track and an incremental code symbol pattern is formed there in a second track parallel thereto. In the absolute code mark pattern, any “n” successive code marks in each instance represent a code word. Each of these code words is present only once in the entire code mark pattern. A code reading device, which can detect “n” successive code marks all at once in a movement direction and in that case scans the incremental code symbol pattern, is provided at a second part movable relative to the first part. If the code reading device is moved along the first part by one code mark position of the absolute code mark pattern, a new n-digit binary code word is read.
In this known device each code word of the absolute code mark pattern defines a specific position of the two parts relative to one another. The length, which is measured in the direction of movement or reading, of the individual code marks and the number of the maximum possible code words establish the maximum length of the measuring path able to be addressed by code words. The resolution capability by which the relative position, i.e. the so-termed position code, expressed in the pseudo random code can be measured depends on the length of each individual code mark. The smaller the length of the code marks, the more accurate the positioning can be. However, reading-off becomes noticeably more difficult with decreasing lengths of the code marks, particularly in the case of high relative speeds.
In the case of use of such an absolute length measuring system for determining the position of a elevator car, such as, for example, the elevator known from German Utility Model G 92 10 996.9, the entire travel path in the travel direction of the elevator car is to be addressed in a gapless manner with coded position details, i.e. the code words of the pseudo random coding. The maximum of the measuring or travel path extent is then, however, limited by the sum of the length of all code marks. A pseudo random coding with multi-digit code words and/or code marks of greater length accordingly has to be provided for long travel paths. However, multi-digit code words necessitate correspondingly complicated code reading devices and evaluating units and this is connected with high costs. With increasing length of the individual code marks, however, the resolution capability diminishes.
In order to avoid errors in reading, the absolute code mark pattern and the incremental code symbol pattern are to be represented in their relative position exactly aligned with one another. This makes the production of a double-track code carrier expensive and moreover necessitates a time-consuming precise mounting. In addition, the code reading device, in particular, of a double-track absolute position measuring system is of large construction, which is undesirable with respect to the limited shaft cross-sectional area available. Furthermore, the travel speed in the case of double-track measuring systems is limited, which is felt to be limiting especially for elevators with large conveying heights.